Organic Broadcaster

Sunn hemp gains popularity as a stress-tolerant cover crop

By Audrey Alwell, MOSES

Sunn hemp has bright yellow flowers that bloom at about 60 days. Due to its rapid growth and nitrogen-fixing capability, sunn hemp is gaining popularity as a cover crop in the Midwest. Photo by Molokai Seed Company

Sunn hemp is a fast-growing, nitrogen-fixing soil builder that Midwest growers are just starting to appreciate as a cover crop. Like other cover crops, sunn hemp (Crotalaria juncea L.) suppresses weeds, reduces erosion, and improves soil tilth. It’s this legume’s ability to thrive in poor soil conditions and withstand drought, heat and wind while producing tons of biomass that is getting growers’ attention.

Sunn hemp is a warm-season annual that grows upright and tall, reaching a height of 4 to 6 feet in 60 days. It has simple, oblong-shaped leaves that are 2.5 to 5 inches long. Branching occurs about 2 feet from the ground or higher if planted in a thick stand as a green manure crop. The plant has a strong taproot and well-developed lateral roots with branched and lobed nodules. Research has shown it suppresses parasitic nematode populations in the soil.

To use as a green manure, the Natural Resources Conservation Service (NRCS) recommends mowing and plowing it under at bud or early bloom stage (around 60 days) when the nitrogen content is high and decomposition will be rapid. If sunn hemp is left to grow longer it becomes fibrous and difficult to turn under. The crop also can be rolled down to create an effective ground cover. While reported totals vary, sunn hemp can produce as much as 6,000 pounds of biomass and 145 pounds of actual nitrogen per acre in 60 days.

Sunn hemp has been used to improve soil in tropical regions for centuries. In countries such as India and Bangladesh, farmers grow sunn hemp for livestock forage—the leaves are 30% protein—and fiber, since the plant has a fibrous stalk when allowed to mature.

Researchers in the U.S. first looked at sunn hemp for production here in the 1930s, but abandoned it when they found the plant won’t set seed above 28 degrees N latitude—only Hawaii, the Florida peninsula and southern Texas are below that latitude. Today’s Midwest farmers find sunn hemp’s inability to form seed appealing—it’s non-invasive and easy to control in the field.

In the early 1980s, the NRCS and the University of Hawaii collaborated on a sunn hemp cultivar called “Tropic Sun.” Seed from the Tropic Sun cultivar can be produced consistently in the U.S. only in Hawaii. Molokai Seed Company, owned and operated by Bradley Sakamoto, is a 35-acre farm in Hawaii that has been growing Tropic Sun organically since 2011. The farm was certified organic in 2014. Sakamoto said Tropic Sun is the only sunn hemp cultivar that has research to prove it’s nontoxic to livestock.

“Other species of Crotalaria contain poisonous alkaloids and, under certain conditions, can be toxic to animals,” he explained. “Forage guys are quickly realizing that animals will eat Tropic Sun virtually over any other crop. This is a huge potential for cattle, goat, sheep, and rabbit farmers, or even guys just wanting to grow a food plot for deer.”

Dion Puzon, Jr., the CEO and publisher of Organic Producer magazine, has just begun a three-year trial of Tropic Sun on OrgPro Farms near Spring Green, Wis. He’s growing sunn hemp on a half-acre plot to compare it with a half-acre plot of clover.

“It’s not necessarily a scientific study,” Puzon said. “I just want to see how the stuff grows.” He’s hoping the sunn hemp will improve his field’s “marginal” soil. “It’s generally sandy soil around here, and takes a lot of work to improve.”

Sunn hemp grows up to 6 feet in Hawaii. Photo by Molokai Seed Company

While Molokai Seed’s Tropic Sun is the only certified organic sunn hemp seed available in the U.S., other seed sources are available. Cover Crop Solutions out of Pennsylvania sources untreated non-GMO sunn hemp seed out of Asia and sells it under the brand name Tillage Sunn™ through a system of distributors nationally. Tillage Sunn™ is another line of Crotalaria juncea. Cover Crop Solutions also offers sunn hemp as part of its TillageMax™ mix with sorghum sudangrass and Tillage Radish®.

Several members of Practical Farmers of Iowa (PFI) experimented with sunn hemp both by itself and as part of diverse cover crop mixes in 2013 and 2014. They sourced non-organic seed from Green Cover Seed in Bladen, Neb., which sells sunn hemp sourced from South Africa. PFI released a four-page fact sheet in March detailing the farmers’ experiences with “quick turnaround cover crops.” (See bit.ly/QuickCovers.) In general, the plant germinated well and provided good biomass and nitrogen.

Rick Hartmann of Small Potatoes Farm near Minburn, Iowa participated in the “quick turnaround” project. He compared sunn hemp with several other cover crops on his vegetable farm and said he liked it.

“It is a good summer cover crop and one of the few to fix nitrogen,” Hartmann said. He recommended it for vegetable growers.

Jon Bakehouse, of Maple Edge Farms near Hastings, Iowa, planted sunn hemp on his own as part of an eight-species cover crop mix after harvesting rye in early July 2013. Rather than turning it under in the fall, he left it in the field over the winter.

“It grew well in our mix and I really liked the way the stalks stood over winter,” Bakehouse said. He said they did have some trouble with the stalks wrapping around the closing wheels when directly planting beans into the stubble, but nothing that would prevent him from planting it again. He noted that it’s definitely a summer cover crop.

“It needs some good growing days, and is expensive, so we don’t usually include it in our fall-planted covers,” he added. Sunn hemp seed can run from $2 to $5 a pound, depending on the quantity ordered, making it cheaper than alfalfa or clover, but more expensive than other legume cover crops.

Planting, Growing Sunn Hemp
Sunn hemp grows best on well-drained soils with a pH from 5 to 7.5. The NRCS recommends planting it in early June when the soil is at least 65 degrees or up to eight weeks before the first fall frost. The seed requires a cowpea-type inoculant to ensure effective nodulation, as some soils may not contain the correct Bradyrhizobium strain.

The NRCS recommends a broadcast rate of 40 to 60 pounds per acre, using the higher seeding rates when the crop will be turned under in 30 to 45 days or when weed pressure is expected to be severe. Cover Crop Solutions, the source for Tillage Sunn, recommends drilling from ½ to 1 inch deep for good seed-to-soil contact. The recommended seeding rate with drilling is 15 pounds per acre.